72 INTRODUCTION TO EVOLUTION 



cems us. In these colonial forms each colony consists of four or more cells 

 each of which is independent of the others except in locomotion; the 

 colony moves as a unit. The species of Gonium shown, for example, con- 

 sists of a colony of four cells bound together by a jellylike substance. 

 When reproduction occurs each cell divides into two, and each of these 

 daughter cells, in turn, divides into two. Thus four daughter colonies of 

 four cells each arise; they are released from the surrounding jelly and take 

 up independent existences. Pandorina and other colonial forms consist of 

 larger numbers of cells — 16, 32, 64, 128, and more. Like Gonium each of 

 these larger colonies begins life as a single cell which divides and sub- 

 divides, stopping when the cell number appropriate to the species has been 

 reached. 



The third squares in both columns of Fig. 4.15 contain spherical ag- 

 gregates of cells, the blastula of embryonic development on the left, a 

 colonial organism called Volvox on the right. Volvox consists of thousands 

 of cells bound together in a spherical colony. Like the blastula in em- 

 bryonic development of higher animals it is a one-layered organism. The 

 bottom squares of both columns (Fig. 4.15) contain two-layered or- 

 ganisms, the gastrula of embryonic development on the left, Hydra, rep- 

 resenting Phylum Coelenterata (jellyfishes, sea anemones, corals, and 

 their kin), on the right. Hydra is a simple fresh-water jellyfish consisting 

 mainly of two layers of cells (ectoderm and endoderm); it is essentially a 

 gastrula with tentacles. Like a gastrula it has a simple digestive cavity 

 with but one opening to the exterior. This pattern is common to the 

 coelenterates, though some differ from Hydra in appearance owing to the 

 presence of great amounts of "jelly" between ectoderm and endoderm. 

 This jelly (mesoglea) does in fact contain cells. 



In the embryonic development of higher animals a third layer, the 

 mesoderm, appears soon after the two-layered stage is attained. In paral- 

 lel manner mesoderm is found in the phylum of animals usually placed 

 next after Phylum Coelenterata in the classification: Phylum Platyhelmin- 

 thes (flatworms), and in all subsequent phyla. 



If the parallelism we have just traced represents a case of recapitula- 

 tion the sequence of stages in the evolution of the metazoa was as follows: 

 (1) single cells; (2) groups of cells aggregated into colonies; (3) spheri- 

 cal, one-layered colonies; (4) two-layered organisms (Phylum Coelenter- 

 ata); (5) three-layered organisms (Phylum Platyhelminthes and higher 

 ones). In other words, the protozoa were ancestral to the coelenterates, 

 which in turn were ancestral to flatworms and higher phyla. Is such an 

 interpretation correct? Since the writings of Haeckel most biologists have 



